It is proposed to examine the origins of cooperativity for two allosteric proteins, for one of which the allosteric effect depends upon an internal conformational change and for the other is linked to a change in the state of association. Among the questions to be considered are the number of discrete conformations involved in the interaction of protein with modifier and substrate, the thermodynamic parameters of the consecutive binding steps, the nature of the induced conformational change, and the mutual spatial arrangement of the protomers.